Blue jeans are manufactured from denim fabric that contains both weft and warp yarns. The fabric is formed by drawing the weft yarn over-and-under through the lengthwise warp yarns that are held in tension on a frame or loom to create the denim cloth. In the fabric, the warp is the lengthwise or longitudinal thread while the weft is the transverse thread. Denim weft yarns are generally white and the warp yarns are indigo dyed. The yarns could be either ring spun or open ended and the indigo dyeing process can be slasher dye or rope dye processes. Rope dyeing is the most common process. Slasher dyeing is less frequently used and is aimed mostly at higher end or smaller production lots of denim. Rope dyeing makes a bundle of yarn, and dyes the whole bundle at once
The dyeing procedure is designed to best apply a ring dyed effect on cotton yarns with indigo, vat, and sulfur dyes. These dye classes require a reduction/oxidation potential (−mV=600 to 800) and high loading of caustic (NaOH, pH approx. 12 to 13) to produce a water dispersible and cotton-substantive dyestuff. The main purpose of ring dyeing is to create a layer of dyestuff on the outside perimeter of the yarn cross section that can be removed when washing steps are performed post dyeing. The stone/enzyme washing creates the desired “salt and pepper” look that retailers and their consumers desire and expect of denim products.
Improved methods for ring dyeing and denim fabric processing would provide a number of advantages. For example, improved ring dyeing would result in a reduction in the use of water, energy, and chemicals in the manufacturing process and even more important a reduction in time to achieve the same wash standard. Further benefits will be the reduced impact on the environment. Furthermore, the delicate balance of variables in traditional dyeing methods are easily disrupted and process improvements can provide improved consistency, reproducibility, and predictability compared to the delicate dyeing processes existing today. Significant benefits in improving the hand sanding or laser abrading process would also be expected. Finally the opportunity to eliminate the use of toxic Potassium Permanganate by achieving the enhanced abrasion typically achieved from Potassium Permanganate but without the use of Potassium Permanganate and with the use of a stone/enzyme/bleach wash.
In a conventional slasher dye or rope dye process, the warp yarn travels through immersion dye boxes as shown in FIGS. 1 and 10, allowing leuco-indigo dye to coat an outer layer of color onto the yarn. This yarn then proceeds into a “skying” or oxidation segment where the soluble leuco-indigo is transformed to the insoluble oxidized blue indigo after exposure to oxygen. The process repeats to continue to build color yield on the perimeter of the yarn.